1. Field of the Invention
This invention relates to a focus condition detecting device of the type which employs a TTL (through the lens) phase difference detecting method, and more particularly to a focus condition detecting device of the type mentioned which has a plurality of focus condition detecting areas. The focus condition detecting device of the present invention is particularly suitable for an automatic focus condition detecting device for a single lens reflex camera on which one of various interchangeable lenses may be selectively mounted when the camera is to be used.
2. Description of the Prior Art
A focus condition detecting device of the TTL phase difference detecting type is widely employed in automatic focusing systems of single lens reflex cameras. In a focus condition detecting device of the type mentioned, a focus condition is determined in accordance with a distance between first and second images of light formed from light from an object which has passed first and second portions of a plane of an exit pupil of an interchangeable lens. In the focus condition detecting device, the position and size of an AF (automatic focusing) pupil which is to be passed by an AF beam of light to be used for detection of a focus condition depend upon designing of a camera body. Where the AF pupil is within an exit pupil of an interchangeable lens, no eclipse will appear in an AF beam of light and no trouble will occur in a focus condition detecting operation. However, the positions or sizes of exit pupils of interchangeable lenses are not constant, and in some cases, an AF pupil may be displaced from an exit pupil so that an eclipse may appear in an AF beam of light. In such an instance, there is a problem that an error appears in detection of a focus condition or in some cases, detection of a focus condition is impossible.
Thus, it has been proposed to prepare a plurality of different types of pairs of focus condition detecting optical sensors and selectively use one of the pairs of focus condition detecting optical sensors in accordance with a fully open aperture value of a lens used (Japanese Patent Publication No. 62-6206). However, since the prior art arrangement does not involve accurate comparison in dimensional relationship or positional relationship between an exit pupil of an interchangeable lens and an AF pupil of a camera body, presence or absence of an eclipse in an AF beam of light cannot be discriminated with accuracy. Besides, it is not at all taken into consideration that an exit pupil may be limited also on the inside thereof as in a cata-dioptric lens nor that the shape of an exit pupil as viewed out of an optical axis may be deformed.
Meanwhile, as interchangeable lenses for single lens reflex cameras having AE (automatic exposure) and AF functions of the TTL type, a large number of lenses have been developed wherein data for automatic exposure such as a fully open aperture value, a maximum aperture value and a focal length and data for automatic focusing such as a lens feeding amount converting coefficient are stored fixedly such that they may be accessed from a camera body. However, no lens has been proposed wherein data of an exit pupil is fixedly stored together with a fully open aperture value. This is because such data of an exit pupil can be found in most cases from data of a fully open aperture value.
In such a focus condition detecting device of the TTL phase difference detecting type as described above, if, for example, a plurality of focus condition detecting frames or distance measuring frames including both on-axis and out-of-axis distance measuring frames are provided and each includes a plurality of distance measuring areas for actually detecting a focus condition therethrough, then the focus condition detecting device will have a very large number of distance measuring areas. In some of such distance measuring areas, optically correct detection of a focus condition may not be attained due to the position or size of the exit pupil of the interchangeable lens or the shape of the exit pupil as viewed from an out-of-axis position. Where such an interchangeable lens is mounted on the camera, it would be waste of time and require an excessively long time for detection of a focus condition if detection of a focus condition is attempted for each of all of the distance measuring areas of each of all of the distance measuring frames of the interchangeable lens.
Further, in the focus condition detecting device disclosed in Japanese Patent Publication No. 62-6206 mentioned above, whether detection of a focus condition is possible or impossible is discriminated depending upon a fully open aperture value of an interchangeable lens. As camera systems have been developed progressively, however, whether detection of a focus condition is possible or impossible may not sometimes be discriminated correctly only from a fully open aperture value of an interchangeable lens. For example, when a composition of photographing is to be decided, it will be convenient if there is a distance measuring area outside an optical axis. In this instance, however, the focus condition detecting optical system will view an exit pupil of the interchangeable lens in an oblique direction comparing with a case wherein detection of a focus condition is conducted in a distance measuring area on an optical axis. Accordingly, it is a problem that the shape of the exit pupil is deformed and consequently it cannot be discriminated correctly from data of a fully open aperture value whether detection of a focus condition is possible or impossible.